Orthogonal Frequency Division Multiplexing (OFDM) is a spread spectrum modulation technique that distributes data for transmission over a large number of narrowband frequency units typically referred to as sub-carriers. The data in each sub-carrier is independently modulated with, for example, Quadrature Amplitude Modulation (QAM) or Phase-Shift Keying (PSK). Essentially, the number of sub-carriers is equal to the size, N, of the Inverse Fast Fourier Transformer/Fast Fourier Transformer (IFFT/FFT) used in OFDM transmitters and receivers.
The background of the invention may be described in the context of an OFDM-based wireless communication system. Essentially, data transmission and reception using OFDM begins with a serial-to-parallel conversion of QAM-modulated symbols (including data for transmission), which are input to an IFFT. At the output of the IFFT, N time-domain samples are obtained. The signal after the IFFT is parallel-to-serial converted, and a cyclic prefix (CP) is appended to the signal sequence. The resulting sequence of N samples (and CP) is referred to as an OFDM symbol. At an OFDM receiver, the CP is removed from the OFDM symbol, and the resulting signal is serial-to-parallel converted and input to an FFT. The output of the FFT is parallel-to-serial converted, and the resulting QAM-modulated signals are input to a QAM demodulator, which outputs baseband data.
In a wireless communication link, a multi-path channel experiences frequency-selective fading. Moreover, in a mobile wireless communication link, a multi-path channel also experiences time-varying fading. However, in a wireless mobile communication system using OFDMA, the system's overall performance and efficiency can be improved by using both time-domain and frequency-selective multi-user scheduling. Thus, by using frequency-domain scheduling for wireless transmissions, the capacity and reliability of the wireless channels can be improved. An approach that introduces frequency-selectivity into a wireless channel in an OFDM network is disclosed in commonly-assigned U.S. patent application Ser. No. 11/327,799 entitled “METHOD AND SYSTEM FOR INTRODUCING FREQUENCY SELECTIVITY INTO TRANSMISSIONS IN AN ORTHOGONAL FREQUENCY DIVISION MULTIPLEXING NETWORK,” filed on Jan. 6, 2006. The subject matter disclosed in U.S. patent application Ser. No. 11/327,799 is incorporated by reference into the present application as if fully set forth herein.
If frequency-domain multi-user scheduling is used in a mobile wireless OFDM communication system, contiguous sets of sub-carriers are allocated for transmission to the mobile users. The total bandwidth of the system is divided into a plurality of sub-bands, and each sub-band includes a grouping of contiguous sub-carriers (e.g., f1, f2, f3, f4). Frequency-domain multi-user scheduling is generally beneficial for systems with low mobility users whose channel quality can be suitably tracked.
If it is desirable to achieve frequency-diversity in a mobile wireless OFDM communication system, the allocated sub-carriers in a band are uniformly distributed over the entire frequency spectrum. In a time-varying mobile wireless channel, the reliability of the transmission can be improved by coding the information over the sub-carriers. However, if the channel's response is flat, frequency diversity is difficult, if not impossible, to achieve. The channel quality of the high mobility users generally cannot be tracked (particularly in those systems where uplink and downlink fading occur independently) because of delays in reporting channel quality. Consequently, the use of frequency-diversity transmissions are preferred for high mobility users.
FIG. 1 depicts a block diagram of an existing OFDM wireless communication system 100, which illustrates a conventional technique used for transmitter antenna beam forming. In the transmitter subsystem 102 of system 100, an OFDM symbol (including N samples of data) 106 is coupled to a plurality of multipliers 108a through 108n (where n represents the nth or final multiplier). Also, beam-forming weights 110a through 110n (g0, g1, . . . , gp) are coupled to respective multipliers 108a through 108n. The (weighted) output of each multiplier 108a through 108n is coupled to a respective transmit antenna 112a through 112n, which transmit the n sub-bands including the OFDM symbol over a radio air interface or channel. An OFDM receiver subsystem 104 receives the n sub-bands in the channel, and outputs the OFDM symbol. Notably, receiver subsystem 104 estimates the complex gains g0, g1, . . . , gp and couples them back to transmitter subsystem 102 via a feedback link 114. These estimated gains are used as the beam-forming weights 110a through 110n in transmitter subsystem 102. A significant problem with this approach is that the feedback information including the complex gains represents a substantial amount of resource overhead, which inefficiently consumes a significant amount of system resources and degrades the overall spectral efficiency of the system involved.